Do not load user or system gitconfig and gitattributes

While doing any kind of git loading, unset HOME variables and set
NOSYSTEM variables so that cgit does not load any settings that a user
may have set for his own /usr/bin/git usage.

This fixes a fatal error introduced with git 1.8, whereupon git would
fatally exit when failing to access particular files.

The result of this is that only repo-local configuration files are
accessed:

zx2c4@thinkpad ~/Projects/cgit $ HOME=/root QUERY_STRING="url=foo/log"
CGIT_CONFIG=tests/trash/cgitrc strace -e access ./cgit >/dev/null
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
access("repos/foo/.git/objects", X_OK) = 0
access("repos/foo/.git/refs", X_OK) = 0
access("repos/foo/.git/config", R_OK) = 0
access("repos/foo/.git/config", R_OK) = 0
access("repos/foo/.git/objects/b3/bafdbf0183f4897ef8b1319cb8c490ed54717e", F_OK) = 0
access("repos/foo/.git/objects/b3/bafdbf0183f4897ef8b1319cb8c490ed54717e", F_OK) = 0
access("repos/foo/.git/objects/b3/bafdbf0183f4897ef8b1319cb8c490ed54717e", F_OK) = 0
access("repos/foo/.git/objects/b3/bafdbf0183f4897ef8b1319cb8c490ed54717e", F_OK) = 0
+++ exited with 0 +++

Reported-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Tested-by: Jason A. Donenfeld <Jason@zx2c4.com>
Tested-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

cgit.c

@@ -459,12 +459,36 @@ static char *guess_defbranch(void)
 
 static int prepare_repo_cmd(struct cgit_context *ctx)
 {
+	char *user_home;
+	char *xdg_home;
 	unsigned char sha1[20];
 	int nongit = 0;
 	int rc;
 
+	/* The path to the git repository. */
 	setenv("GIT_DIR", ctx->repo->path, 1);
+
+	/* Do not look in /etc/ for gitconfig and gitattributes. */
+	setenv("GIT_CONFIG_NOSYSTEM", "1", 1);
+	setenv("GIT_ATTR_NOSYSTEM", "1", 1);
+
+	/* We unset HOME and XDG_CONFIG_HOME before calling the git setup function
+	 * so that we don't make unneccessary filesystem accesses. */
+	user_home = getenv("HOME");
+	xdg_home = getenv("XDG_CONFIG_HOME");
+	unsetenv("HOME");
+	unsetenv("XDG_CONFIG_HOME");
+
+	/* Setup the git directory and initialize the notes system. Both of these
+	 * load local configuration from the git repository, so we do them both while
+	 * the HOME variables are unset. */
 	setup_git_directory_gently(&nongit);
+	init_display_notes(NULL);
+
+	/* We restore the unset variables afterward. */
+	setenv("HOME", user_home, 1);
+	setenv("XDG_CONFIG_HOME", xdg_home, 1);
+
 	if (nongit) {
 		const char *name = ctx->repo->name;
 		rc = errno;

ui-commit.c

@@ -37,7 +37,6 @@ void cgit_print_commit(char *hex, const char *prefix)
 	}
 	info = cgit_parse_commit(commit);
 
-	init_display_notes(NULL);
 	format_display_notes(sha1, &notes, PAGE_ENCODING, 0);
 
 	load_ref_decorations(DECORATE_FULL_REFS);

ui-log.c

@@ -403,7 +403,6 @@ void cgit_print_log(const char *tip, int ofs, int cnt, char *grep, char *pattern
 		commit->parents = NULL;
 	}
 
-	init_display_notes(NULL);
 	for (i = 0; i < cnt && (commit = get_revision(&rev)) != NULL; i++) {
 		print_commit(commit, &rev);
 		free(commit->buffer);